The use of glasses containing metal, in particular the elements of the alkali and alkaline-earth metals, is impossible or only limitedly possible in many components since, for example by diffusion or exchange processes, alkali and alkaline-earth metal ions emerge from the glass and exert undesired effects.
This particularly affects medicaments which are stored in glass vessels containing alkali metal. Especially in the case of aqueous solutions, leaching of the glass surface takes place so that the medicament becomes contaminated with alkali and alkaline-earth metal ions. In the case of display glasses as well, the diffusion of alkali and alkaline-earth metal ions into applied semiconductor structures can even lead to failure of the display or at least individual pixels. This particularly affects the case in which high-temperature steps are carried out during production of the display, which significantly accelerate the diffusion of alkali and alkaline-earth metal ions. In many cases, it is therefore necessary to reduce the alkali and alkaline-earth metal concentration at least in the superficial region of a substrate.
In the case of glass, for example, this may be carried out during hot forming. For instance EP 1 138 639 discloses a method for producing glass tubes, in which the tubes are hot-blown in a gas atmosphere containing oxygen. This leads to reduction at least of sodium ions on the glass surface. A disadvantage with this method, however, is that alkali metal ions can reach the surface again by diffusion if further hot forming steps take place in the production chain.
Another possibility is to wash glass containing alkali metal with aqueous solutions, acids or sulfate solutions. It is known in practice from pharmaceutical packaging, for example, to rinse the surface of glass with an ammonium sulfate solution in order to reduce the metal ion content. Such chemical cleaning and leaching methods, however, entail the risk of contamination. A further risk is that this treatment generally attacks and damages the glass network at least in the surface region, so that the strength of the glass is reduced and the risk of the glass cracking is therefore increased.
As an alternative to the leaching of substrates, barrier layers may be applied as diffusion barriers for alkali and alkaline-earth elements. Such barrier layers are relatively expensive and cannot be applied in all cases. For example, further hot forming of the preform may lead to destruction of the barrier layer.
It has furthermore been found that according to the prior art, it is often impossible to reduce the concentration of alkali and alkaline-earth metal ions sufficiently in the superficial region.